254 CHEMICAL ENGINEERING
greater overloading with a trommel than with a shaking screen. Where a trommel is overloaded there is confusion of the limits of screening, so far as the eye can judge both in the under and the oversizes. The undersizes will contain particles of very nearly the size of the apertures and the oversizes will have much small particles and fines. On a shaking screen unless excessively overloaded few large particles will report in the undersizes and an inappreciable amount of fines will be left in the oversize.
Wet Screening with Trommels.—It has already been stated that the application of water to a flat screen to assist in screening is a failure, for it drains through leaving a sticky non-progressive mass behind it. Within certain limits this is not true of the trommel. The wash water for these screens is applied in a sheet on the upcoming side and while much of it falls without the trommel and does no good a certain proportion passes through the screen and flows down under the bank where it is held by the particles as it would be by a sponge and keeps the grains from cohering, lubricates the grains in their passage through the apertures and carries undersize through the opening by direct flowage. This effect cannot be obtained on a flat screen for the water from sprays drains through as fast as applied.
Effective use of water on a trommel becomes less as the size of the particles is diminished on account of coherence of the bank and ultimately the smearing over and. complete blinding of the aperture unless an excessive amount of water is used giving an effect equal to suspending the material fed in "water. Below -k^-in. size the application of water tends rapidly to increase the waterlogging of the bank until finally a point is reached where there is practically none of the rotation so necessary for good screening, the bank swinging up and down as a damp clinging mass.,
Range of Use of Trommel.—On dry material the rotation begins to be slow and defective at about a %-in. size. The field of the trommel is for capacity screening between J^ and 2J£ in. Above the latter size well designed revolving grizzlies while competing on the score of wear and tear with well-designed balanced grizzlies of the type where the material fed is moved bodily along instead of being shaken along do not compete with them on the scores of first cost and capacity.
Within its field and range the trommel has the following advantages:
(1) An almost perfect balance; (2) ability to do fairly good work even when heavily overloaded; (3) It takes care of fluctuations in feeding better than any other screening device; (4) as compared with flat shaking screens there is loss tendency to blind, due to the fact that there is no motion tending to jam a grain down into the apertures as there is with ordinary shaking screens. In this respect they are not superior to gyratory or flat screens which advance the material fed without any tendency to jam the particles into the apertures. The trommel can usually be easily beaten out while the screen is revolving with a piece of belting fastened to a handle. This simple method of relieving blinding cannot be practiced with ordinary types of screens; (5) with flat shaking screens of any width it is necessary in order to secure the best effect to distribute the feed evenly over the width of the screen, while with revolving screens it is merely necessary to discharge the feed into the upper end of the screen; (6) the trommel may be used as a wet screen.
The principal disadvantage of the trommel is the wear and tear on the supporting parts and the time and difficulty of making screen section changes.
Capacity of Trommels.—A disadvantage often cited against shaking screens by the unthinking is that only part of the screen is in use at any one time but on